Three-dimensional microscopy reveals unusual cells

Although Fuji apples generally have good storage properties, they tend to turn brown inside when in long-term “modified atmosphere” storage if the temperature, oxygen and carbon dioxide levels are not tightly controlled. Researchers at the Institute of Food Research, part of the Norwich BioScience Institutes in the UK, may have found out why, using three-dimensional light and electron microscopy.

With an Olympus microscope, they captured images of unstained thin apple slices in bright-field mode and under UV light using the natural autofluorescence of chlorophyll (red) and vacuole contents (blue). They also captured images of slices stained for starch with a mixture of iodine and potassium iodide. They obtained a closer look at the apples with a Leica scanning electron microscope operated at 10 kV.

Researchers found unusual branched tubular cells in Fuji apples and related varieties using three-dimensional bright-field (left), autofluorescence (middle), scanning electron microscopy (right) and stained fluorescence (not shown). Reprinted with permission of Postharvest Biology and Technology.They discovered unusual branched tubular cells that principal investigator Mary L. Parker called “callus hairs,” occupying the spaces between cells where air is supposed to flow. She believes that the callus hairs not only could impede air flow but, because they use vital oxygen and output carbon dioxide waste, they also could create local pockets where the gas composition triggers spoilage.

It is important to understand which growth conditions or orchard management decisions affect callus hair development in ripening Fuji apples, Parker said. So far, callus hairs have been found in Fuji apples from around the world, so their presence could be used to verify the authenticity of dried apples labeled Fuji.

The scientists also have been investigating the composition of callus hairs. So far, these studies suggest that the hairs are rich in plant nutrients but also could produce allergens.